This invention relates to optical dispersion compensation and optical pulse manipulation using Bragg reflection gratings.
The use of Bragg reflection gratings is known for causing wavelength-dependent delays, which compensate for dispersion effects. Many optical materials exhibit different responses to optical waves of different wavelengths. Chromatic dispersions, often simply referred to as xe2x80x9cdispersionxe2x80x9d, is one well-known resulting phenomenon, in which the index of the refraction of a medium is dependent on the wavelength of an optical wave. Dispersion can cause optical waves of different wavelengths to travel at different speeds in a given medium, since the speed of light is dependent on the index of refraction. The dispersion of optical materials in general relates nonlinearly to the wavelength.
In many applications, an optical signal is composed of spectral components of different wavelengths. For example, a single-frequency optical carrier may be modulated in order to impose information on the carrier. Such modulation generates modulation sidebands at different frequencies from the carrier frequency. Also, optical pulses, which are widely used in optical data processing and communication applications, contain spectral components in a certain spectral range. The dispersion effect may cause adverse effects on the signal due to the different delays on the different spectral components.
Dispersion in particular presents obstacles to increasing system data rates and transmission distances without signal repeaters in either single-channel or wavelength-division-multiplexed (xe2x80x9cWDMxe2x80x9d) fiber communication systems. Data transmission rates of tens of Gbit/s may be needed in order to meet the increasing demand in the marketplace. Dispersion can be accumulated over distance to induce pulse broadening or spread. Two adjacent pulses in a pulse train thus may overlap with each other at a high data rate due to dispersion. Such pulse overlapping can often cause errors in data transmission.
There have been various proposals for overcoming the dispersion effect. This invention is concerned with the use of a Bragg grating. Such gratings are known both with linearly chirped (i.e. varying) grating periods and with non-linearly chirped graft periods, in order to achieve the desired spectral response along the length of the optical carrier (fiber or waveguide).
A spectral component in an optical signal with a wavelength satisfying a Bragg phase-matching condition is reflected back from a Bragg grating. Other spectral components are transmitted through the grating. The Bragg phase-matching conditions at different positions in the fiber grating are differentiated by the chirping of the grating period, so that the resonant wavelength of the fiber grating changes with the position. As the grating period increases or decreases along a direction in the fiber grating, the resonant wavelength increases or decreases accordingly. Therefore, different spectral components in an optical signal are reflected back at different locations and have different delays. Such wavelength-dependent delays can be used to negate the accumulated dispersion in a fiber or waveguide link.
To use a chirped Bragg grating, an optical circulator is typically used to couple the input optical signal to the grating and to route the reflected signal. An optional optical isolator may be placed at the other end of the grating to reject any optical feedback signal.
A common method of writing a Bragg grating uses a phase mask in the form of an etched fused silica grating, to form a light interference pattern, which then illuminates a photosensitive optical carrier to alter the refractive index characteristics. The phase mask is generally made using holography or electron beam lithography. In order to write a fiber grating of pitch p, a phase mask of pitch 2p is written and illuminated at normal incidence with coherent radiation of an appropriate UV wavelength to photoactivate the fiber.
One of the main problems with this method of production is that any errors on the phase mask are translated into errors of some form in the actual fiber grating itself. Holographic writing techniques are not ideally suited for chirped gratings, because many grating lines are written simultaneously, with overlap between successive writing operations. Phase masks produced using electron beam lithography have extremely good precision over many lines, but there is a line placement error on each of the individual grating lines. This is because each line is exposed individually by the electron beam process. This does, however, make the process particularly suited to the manufacture of chirped gratings. Relative movement is ultimately controlled by an interferometer device, which in the past has imposed a maximum accuracy of around 5 nm, although recent developments are enabling increased positioning accuracy. The electron beam itself has a width of approximately 150 nm-200 nm, and a typical grating pitch of the phase mask is 1070 nm. The random placement error leads to imperfections in the interference pattern written into the fiber that manifests itself as a rise in the out-of-band reflectivity of the reflection spectrum of the fiber grating and also as a source of delay ripple in the delay characteristics of the grating.
Line placement errors also arise in phase masks manufactured using other techniques, for example hybrid holographic techniques, which are frequently used for generating phase masks with nonlinear chirp functions.
A phase mask is composed of stitched sections or segments typically 0.1 to 1 mm in length, determined by the field size of the e-beam machine. These stitched segments produce a stitch error which is highly periodic and leads to side lobes in the fiber grating reflection spectrum. These are highly problematic for similar reasons to those described for the random line placement error above. This error, in older e-beam machines was typically 10-20 nm, but has been improved to less than approximately 5 nm using more recent technology. Still more sophisticated slurring techniques have recently been reported which distribute the stitch over a region of mask rather than occurring all at one point and have led to improved side lobe suppression.
Typically, a phase mask produces a fiber grating up to 100 mm in length. If a longer grating is required then a number of gratings patches are joined together. In order to obtain a single uninterrupted continuous grating, the precision of the patch positions must be much better than the grating pitch. In practice, even with highly sophisticated alignment techniques, there is always a stitch-error which leads to severe degradation of the grating performance.
According to the invention, there is provided a method of writing a diffraction grating into a photosensitive optical transmission medium, comprising the steps of:
using a first phase mask to generate an interference pattern in a second phase mask substrate and using photolithographic techniques to create a second phase mask;
using the second phase mask to write a diffraction grating into a photosensitive optical transmission medium.
The method of the invention enables the line placement in a phase ask to be improved by averaging out the line placement errors. This is achieved by carrying out a multiple-stage phase mask creation process, by a first phase mask is used to create holographically a second phase mask, rather than being used to write the grating into the fiber or waveguide.
The first phase mask may be manufactured using electron beam photolithographic techniques.
The medium preferably comprises an optical fiber and the first phase mask preferably a chirped grating period.
The generation of an interference pattern in the second phase mask substrate uses a UV beam which illuminates the first phase mask perpendicularly to the longitudinal axis of the first phase mask. This means that the pitch of the written grating is half of the pitch of the phase mask used to write the grating. Therefore, the first phase mask preferably has a pitch of 4 times the pitch of the desired grating, as a result of the two stage process.
The invention also provides a method of writing a diffraction grating into a photosensitive optical transmission medium, comprising the steps of:
(i) using a first phase mask to generate an interference pattern in a second phase mask substrate and using photolithographic techniques to create a second phase mask;
(ii) carrying out at least one further phase mask manufacture process, the at least one further phase mask process starting with the a previously created phase mask and producing a further phase mask by generating an interference pattern in a further phase mask substrate and using photolithographic techniques, the at least one further process resulting in a final phase mask; and
(iii) using the final phase mask to write a diffraction grating into a photosensitive optical transmission medium.
In this method, a repetitive phase mask writing process is adopted, each stage in the process improving the errors in line placement. When using normal exposure of the mask, the pitch of the first mask will be 2(n+2).p, wherein n is equal to the number of further phase mask manufacture processes and p is the desired pitch of the fiber grating. The case of n=0 corresponds to the two-gage process above with the mask having a pitch of 4p.
According to a second aspect of the invention, there is provided a phase mask for writing a diffraction grating with a pitch p into a photosensitive optical transmission medium, comprising a phase mask written using electron beam lithography having a pitch of 2(n+2).p, wherein n is greater than or equal to 0.
The invention enables the delay ripple in a chirped grating to be reduced, and according to a third aspect of the invention, there is provided a chirped diffraction gating written using a phase mask and having a mean group delay ripple of less tan 5 ps, and preferably below 2.5 ps.